They all have a similar meaning - but I use slim for someone who is normal weight and thin for someone who might weigh slightly under average. Skinny would be the same as slim or thin but could be said as a soft criticism and scrawny as a harsher criticism of someone's thinness - but not cruel or unkind. Slim-thin-skinny-scrawny.

Skinny has a similar meaning to thin but a thin person can still look healthy. A skinny person is to thin for their height and can look unhealthy. A scrawny person is small in height and build. They can also seem unhealthy but not all the time. Help this helps. Have fun